Avoiding Arduino millis() Rollover Bugs

If you are using the millis() function to measure time on an Arduino, you might be tempted to do something like this:-

unsigned long timer_expires_at = millis() + SOME_DURATION; while (millis() < timer_expires_at) { [do other stuff while waiting for the timer to expire] } [do stuff that should happen after the timer expires]

For the most part, that will work well. But after around 49.7 days (4,294,967,296 a.k.a 2³² milliseconds, to be exact), you might run into a problem. If millis() + SOME_DURATION exceeds 4,294,967,295, it will rollover and your comparison will match prematurely. Of course, you would have to be pretty unlucky for this to occur in practice, but - as Mary Chapin Carpenter put it - Sometimes you're the Louisville Slugger, sometimes you're the ball.

I had plans sketched out for a small C++ class to handle this elegantly when I decided to do a quick bit of Googling to see if I was reinventing the wheel entirely here and I came across the most beautiful simple, and elegant solution to the problem in https://arduino.stackexchange.com/questions/12587/how-can-i-handle-the-millis-rollover/12588#12588. It is a long, detailed reply and well-worth reading, but the gist of it is to just change the logic very slightly:-

unsigned long timer_started_at = millis(); while (millis() - SOME_DURATION < timer_started_at) { [do other stuff while waiting for the timer to expire] } [do stuff that should happen after the timer expires]

It should be pretty clear how this avoids the problem after ~49.7 days, but it may be less obvious why it doesn't just shift the problem to the other end.  Imagine that your Arduino has been powered on for 30 seconds, so a call to millis() will return a value of 30,000. You want to wait for 40 seconds, so you set SOME_DURATION to 40,000. Subtracting 40,000 from 30,000 in the condition of the while() loop will indeed cause a rollover (30,000 - 40,000 mod 4,294,967,296 = 4,294,957,296), but the result of that rollover will still satisfy the condition in the way we want.